Weirs have been used for many years to control the level of water in aqueducts, channels or canals. Early weirs were simple gates which were lowered or raised to control the level of water upstream from the weir, to prevent flooding and overtopping of the canal's banks.
These weirs had many disadvantages, such as the need to continually monitor the level of water upstream from the weir in order to determine whether the gate should be closed or opened further. Furthermore these weirs were cumbersome to operate, often requiring two individuals on either side of the weir to raise or lower the gate. Moreover as many of these weirs were constructed of wood or metal, the gates often jammed due to material failure such as warping or metal deterioration. Also due to such material failure, water leakage between the gate and weir, and from the weir itself, often occurred, leading to the uncontrollable release of water through the weir. Finally, the elevation of these weirs could not be altered in response to lower or higher upstream water levels as they were permanent and immovable structures.
More recently weirs have been developed which may either be installed in an aqueduct, channel or canal or else float on a body of water. The latter type of weir is often used as "skimmer," for the removal or separation of surface debris, films, scum or secondary liquids from a primary liquid body.
These weirs incorporate an AMIL type gate which is pivotally centered on an axis of rotation with ballast containers opposite the gate. These containers may be filled or emptied. These gates of the AMIL type, which are disclosed in U.S. Pat. Nos. 3,643,443; 3,683,630; and 4,027,487 to Alexandre, maintain the upstream water level at the elevation of the axis of rotation. Once the gate is installed and the ballast containers counterpoised, the torques generated by the hydraulic thrust of the upstream water on the gate and the weight of the gate are equal and the gate opens and closes in relation to the elevation of the pivot axis and the upstream water level.
By design, these gates do not lend themselves to convenient adjustment of the controlled upstream water level. Once the gate is installed and balanced, the controlled water level can only be changed by shifting ballast from one container to the other and the amount of adjustment is very limited. Therefore if the upstream water level decreases below a certain level, the AMIL type gate is inoperative.